Bimetallic thermostat material



Dec. 13, 1927. 1,652,556

P. H. BRACE BIMETALLIC THERMOSTAT MATERIAL FiLed March 12, 1925 Thermal 'expansivify.

0 I00 260 .360 460 560 6 00 760 Temperature in degrees C.

WITNESSES: INVENTOR Porter H. Brace Patented Dec. 13, 1927.

UNITED STATES PATENT OFFICE.

PORTER E. BRACE, 0F WILKINSBURG, PENNSYLVANIA, ASSIGNOR TO WESTINGHOUSE ELECTRIC & MANUFACTURING COMPANY, A CORPORATION OF PENNSYLVANIA.

BIMETALLIC THERMOSTAT MATERIAL.

Application filed March 12, 1925. Serial No. 14,970.

My invention relates to temperature controlled devices and particularly to bimetallic thermostats.

One object of my invention is to provide a bimetallic thermostat, the elements of which shall have a relatively large difference be tween their respective temperature co-eflicients of expansion, and that shall be composed of material capable of being subjected to relatively high temperatures without deterioration in its operating characteristics.

Another object of my invention is to provide a bimetallic thermostat comprising two closely related alloys that shall however, have dissimilar temperature coeflicients of expansion.

Another object of my invention is to provide a bimetallic thermostat comprising two closely related alloys, the composition of one or both of which may be varied in accordance with the desired temperature limits within which the said device is to operate.

In practicing my invention, I provide a bimetallic thermostat that is made u of an irreversible and a reversible nicke steel. The amount of nickel in the reversible nickel steel varies substantially in accordance with the upper temperature limit Within which it is desired that the thermostat shall operate.

In the single sheet of drawings, I have illustrated temperature-expansivity curves of alloys of nickel steel containing different percentages of nickel, which alloys I employ in making bimetallic thermostats.

The numeral 11 designates a curve of thermal temperature coefiicient of a nickel steel alloy containing or comprising sub stantially 26% of nickel. This curve 15 substantially a straight line between the limits of zero and of 500 C. and it may be noted that the value of the temperature coeflicients of expansion increase with an increase in the temperature.

A nickel steel alloy containing substantially 26% of nickel is an alloy that is within the range of composition covered by the so-called irreversible nickel steels. However, the expansion of such a nickel steel is reversible between certain temperature limits, which limits are dependent upon the nickel content, provided the material has been heat treated at a temperature above its transformation point. For 26% nickel steel the limit is approximately C. at the lower end of the scale and above 600 C. to

the temperature coefficients of expansion will be substantially those indicated by the curve designated by the numeral 11. The lower temperature limit may be varied by varying the 1percentage of nickel in the alloy, and it may e materlally reduced below the figure above given by increasing the percentage of nickel in the alloy.

The heat treatment of the irreversible nickel steels consists in heating them to a temperature above the A transformation point. In pure iron this occurs at a temperature in the neighborhood of 900- C. This transformation point is progressively lowered by increasing quantities of nickel and the temperature at which it occurs on heating becomes more and more widely separated from the temperature at which it occurs on cooling. It is this temperature hysteresis of the thermal effect which gives rise to the appellation irreversible. For nickel contents above approximately 30% the transformation disappears and the length-temperature curves on heating coincide very closely with those on cooling, hence these alloys are called reversible alloys. If an irreversible alloy is heated above its upper transformation point and not subsequently cooled below its lower transformation point, it will expand and contract in a nearly reversible manner but at a much higher rate than normal. It is this property of the irreversible nickel steels that is taken advantage of in order to obtain relatively high and reversible thermal expansivity.

The second element of my heat responsive device comprises a nickel steel alloy having a different amount of nickel, and in general, the percentage of nickel in the second ele ment will exceed 30% and the numeral 12 indicates a curve illustrating the tempera ture coefficient of expansion of a nickel steel alloy having approximately 42% nickel. This material is within the range of compositions covered by the so-called reversible nickel steels, and it may be noted further that the temperature coefficient of expansion of the 42% nickel steel approaches that of the 26% nickel steel when the material is subjected to a temperature substantially in excess of 300 C. The curve of temperature coeflicients of expansion up to 300 C. indicates that the values thereof decrease somewhat with increased temperatures up to 300 C. and are different than those possessed by the 26% nickel steel.

As indicated by the curves 11 and 12,

. the temperature coefficient of expansion of the 26% nickel steel is substantially three times that of the 42% nickel steel for temperatures between zero and substantially 300 C. Hence, it is obvious that a heat-responsive device made in the form of a bimetallic thermostat in which two bars one of 26% and the other of 42% nickel-steel are suitably fixedly connected at their ends or are connected over their entire abutting faces or surfaces, as by welding, will have relatively large deflections for given changes of temperature when the heat-responsive device is mounted and employed in the usual manner well known in the art.

A heat-responsive device comprising a 26%and a 42% nickel steel will, therefore, 7

be operative for a temperature range of substantially zero to 300 C.

If it is desired to increase this temperature range, particularly at the upper limit thereof, I may emplo a nickel steel for one of the elements t athas a relatively higher percentage ofnickel. The temperature coefficients of expansion of a 57% nickel steel are indicated in the curve desig nated bythe numeral 13. The temperature coeflicients of expansion are relatively greater than those for the 42% nickel steel at the lower temperatures but are lower at temperatures substantially above 350 C. The values of the coefficients are also less subject to variation and a heat-responsive device embodying metallic elements comprising alloys having 26 and 57% respec tively of nickel may be employed at temperatures up to approximately 500 C.

While the nickel steel alloy for 57 nickel is irreversible, the transformation points occur at temperatures beyond 500 or even 550 C. and hence a heat-responsive device comprising these metallic elements will operate satisfactorily within the tempera ture range of 0 C. to within 500 C.

As it is important to retain a relatively high elastic limit of the metallic elements at the high temperatures to which they will be subjected, I may incorporate other metals in the respective alloys. I may use, for in- The device embodying my invention thus comprises a plurality of metallic elements that are combined or connected in order that their respective temperature coefficients of expansion ma cause a deflection of one part of the device relatively to the other part, 'in a manner well known in the art. The metallic elements are closely related in that they comprise nickel steel alloys of different nickel content, the percentage of nickel being increased in accordance with the maximum temperature at which it is desired that the heat-responsive device shall operate properly. Auxiliary metals, in relatively small percentages, may be added, to increase the elastic. limit of the alloys at the higher temperatures.

Various modifications and changes may be made Without departing from the spirit and scope of the invention, I desire, therefore, that only such limitation shall be placed thereon as are imposed by the prior art.

I claim as my invention:

1. A heat-responsive device consisting of two metallisr elements of nickel steel alloys, the nickel to iron ratio in one of said elements bein less than .3 and being more than .3 in the other element.

2. A heat-responsive device comprising two metallic elements each consisting of a nickel steel alloy, the nickel to steel ratio in one of said elements being less than .3 and being more than .3 in the other element, and

both elements containing less than 15% of metals tending to increase the elastic limit of the elements at temperatures above 300 C.

3. A heat-responsive device comprising a bimetallic structure, each element of which is a nickel steel alloy and such percentages of other metals to so change the transformation points 'of the component nickel steel alloys as to permit them to be employed over any desired temperature ranges.

In testimony whereof, I have hereunto subscribed my name this 9th day of March, .1925.

PORTER H. BRACE. 

